Add first, last and subspan to SkEnumerate

Change-Id: I0cfd2f710acdb8070325b950a4502c99a668c8b2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/262943
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed